The present invention relates to a sheet feeding apparatus of cassette type for use in copying machines.
In a conventional electrophotographic copying machine, a sheet feeding roller, which is disposed above a cassette, is brought into pressure contact with a leading edge portion of the top sheet of a stack of sheets held in the cassette, and the sheet feeding roller is rotated. At the same time, the sheets are individually separated from the stack of sheets and fed into a predetermined portion by corner separators bringing the leading edge portions of the top sheet upwards elastically. In this sort of conventional sheet feeding apparatus, a stack of sheets is placed on a bottom plate disposed in a cassette and the bottom plate is elevated by rotating a buttom plate pushing member receiving the bottom plate, with one end of the bottom plate being a fulcrum for the elevation of the bottom plate, so that the top sheet of the stack of sheets is brought into contact with the sheet feeding roller, with a predetermined pressure and friction necessary for shifting and feeding the sheets individually from the stack of sheets. In this sheet feeding apparatus, a top portion of the pushing member can enter and retract from a hole of the bottom of the cassette. When the cassette is detached from and attached to the copying machine, the top portion of the bottom plate pushing member is retracted from the cassette, and at the same time, the bottom plate is moved downwards, so that the upper surface of the sheet is released from the pressure contact with the sheet feeding roller.
However, in the conventional sheet feeding apparatus, normally, that is except when the cassette is attached or detached, the upper surface of the sheet is in pressure contact with the sheet feeding roller and such condition may last for a long time. In particular when the ambient humidity is high, a sort of wave is formed in a portion of the stack of sheets in contact with the sheet feeding roller. When the sheets are deformed like this, each sheet does not contact uniformly and closely with a photoconductor, in the case of a electrophotographic copying machine, and consequently the image is not transferred uniformly from the photoconductor to each sheet, with untransferred portions being left behind, and other problems may occur. In an electrophotographic copying machine of direct image formation type using zinc oxide coated photosensitive paper, a zinc oxide layer which constitutes the uppermost layer of the zinc oxide coated photosensitive paper may be scratched by the sheet feeding roller and the trace of the scratch may be left on the copy image. As the pressure of the sheet feeding roller applied to the sheet and the time for the sheet feeding roller to be in pressure contact with the sheet increase, the scratch may become conspicuous. Furthermore, in the conventional sheet feeding apparatus, since the above-mentioned bottom plate pushing member is retracted from the cassette whenever the cassette is detached from the copying machine, a troublesome lever operation from the outside of the copying machine is required.